PPRuNe Forums - Universal joint

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you guys did so well on that last test, I decided to give you another opportunity to excel.

It it seems to me that the teetering head is a poorly designed universal joint. Yes it is cheap and easy, and the range of motion is less than 10 degrees. And it works.

But considering efficiancy and binding; has anyone ever used a constant velocity joint in place of the universal?

Let the education begin

thanks
bill

Yep I can see how a double hooks or a true CV joint would be a floppy mess until up to speed anyway.

I can feel myself getting smarter here by the minute.

Thanks

Bill

Quote:

Originally Posted by SansAnhedral (Post 10251992)

A CV drive would be used on a multi-bladed stiff-in-plane gimballed rotor. To wit, the only craft with this arrangement are tiltrotors.

......or an AS350/H125.

Quote:

Originally Posted by heliduck (Post 10252277)

......or an AS350/H125.

AS350 certainly is not stiff in plane nor gimballed with Starflex.

https://blog.aopa.org/aopa/wp-conten.../05/photo1.jpg

Quote:

Originally Posted by discap (Post 10252019)

Yep I can see how a double hooks or a true CV joint would be a floppy mess until up to speed anyway.

I can feel myself getting smarter here by the minute.

Thanks

Bill

Production CV drives on tiltrotors also use what is referred to as a hub spring, which is a very large spherical elastomeric that acts as a flapping centering "spring" and also what transmits the thrust to the mast in most cases.

The OP never asked for information on gimballed or stiff in plane engineering, he/she asked if anyone had used a constant velocity joint instead of a universal. The star flex arrangement is a constant velocity joint.

Yes heli duck Cv vs universal particularly in a 2 bladed system.

Its “he” by the way.

Bill

Quote:

The star flex arrangement is a constant velocity joint.

The angle between the drive shaft and the rotor hub doesn't change - how is that a CV joint? Only the blades and the plates change angle with respect to the MR shaft, not the hub.

Just some Wiki help so we can all understand the uncommon drive options not found on helicopters very much. I didn't realize there were so many

https://en.wikipedia.org/wiki/Constant-velocity_joint

When referring a starflex as a CV joint, I can see the loose connection, much the same as the OP referred to a teetering head as a Universal joint. Both are generalizing the function/connection.

As I said, no change in angle between driving and driven elements - ergo not a CV joint

Quote:

Originally Posted by heliduck (Post 10253240)

Except you don't use a constant velocity joint, or any angular joint at all, unless you have a stiff-in-plane gimballed rotor. In all other cases the hub is rigidly fixed to the mast and the angular changes for the rotor thrust vector are performed through discrete hinges or flexures along the blade axes.

Quote:

Originally Posted by [email protected] (Post 10253911)

As I said, no change in angle between driving and driven elements - ergo not a CV joint

Agreed, this is precisely the point. There is no angular joint between the rotor hub and the mast whatsoever with starflex, constant velocity or not.

Nobody would consider a wheel bolted to the end of a straight rear drive axle on a truck to be a "constant velocity joint".

Quote:

Originally Posted by [email protected] (Post 10253670)

The angle between the drive shaft and the rotor hub doesn't change - how is that a CV joint? Only the blades and the plates change angle with respect to the MR shaft, not the hub.

if the velocity of the blades don’t change (it remains constant) despite the angle between them & the mast changing then it is a constant velocity joint, hence the name.
To elaborate I’ll refer back to the OP’s comparison of a universal joint & use a truck as an example - although it might appear that the drive shaft on a truck rotates at a constant speed, this is only the case when both drive shafts either side of the universal joint are inline. When the rear axle moves & the drive shafts are no longer inline, the velocity of the driveshaft rotation fluctuates twice/revolution as the centre cross of the universal joint moves. This fluctuation is minute at small driveshaft misalignment angles & increases as the misalignment increases.
The terminology of “constant velocity” refers to the driven portion of a power train, the “joint” refers to the ability of the driven portion to operate out of alignment with the driving section.
The disc does not remain aligned with the mast & the velocity of the blades remain constant, makes sense to me so how is it not a CV joint?
Back to original question, I’m not aware of any CV arrangements for 2 bladed Helicopters. Maybe the teetering head doesn’t create the blade velocity fluctuations like a universal joint would so it’s not worth the engineering costs to change it? If the teetering head doesn’t create velocity fluctuations then by definition it is a constant velocity joint & most of this discussion is irrelevant. I would have thought that an elastomeric head would have been an option, are there any around?

The blades are attached to the hub by the plates and the elastomeric bearings and they do move with respect to the mast BUT, the hub is what is directly driven by the mast and that angle doesn't change.

Quote:

Originally Posted by [email protected] (Post 10254349)

the hub is what is directly driven by the mast and that angle doesn't change.

Spot on again. Our anatine friend just doesn't get it.

someone pulled out a thesauraus.....

Quote:

Originally Posted by GrayHorizonsHeli (Post 10257390)

someone pulled out a thesauraus.....

Ornithology/helicopters - similar hobbies in some respects. Usually prefer they dont mix.